In general, offshore well operations have been conducted from floating vessels, semi-submersible vessels, and platforms having rigid structural legs resting on the sea floor. Each of the above types of offshore apparatus provides certain advantages and disadvantages depending upon the location of the well operation. The quantum and order of disadvantages of such apparatus tend to increase as the water depth at a selected location becomes greater. The disadvantages include both operational or functional disadvantages as well as economic or more costly disadvantages. For example, a rigid fixed leg platform may be practical and feasible in shallow waters of up to about 300 to 400 feet. Such a platform is impractical and difficult to build and install in deep waters in the order of 600 to 2,000 feet. Floating vessels including semi-submersibles are subject to movement from wind and wave forces which impose three dimensional forces on the vessel in relation to a fixed stationary subsea installation. Maintenance of a vessel's position relative to such a fixed subsea installation and the adaptability or accommodation of well equipment to small changes of vessel position is very difficult and presents numerous problems. Changes in movement of a vessel with respect to a riser connection is one example.
To avoid many of the disadvantages of such prior proposed offshore well apparatus, platforms identified as tension leg platforms have been proposed because the installation of such a platform provides many advantages in deep water well operations. Tension leg platforms utilize cables extending from an anchor at the sea bottom to the platform, the buoyant support means of the platform being controlled so that a tension force is imparted to the cables which will prevent the cables from becoming slack under all conditions of loading and operation of the platform. Such tension leg platforms have been disclosed in U.S. Pat. Nos. 3,154,039, 3,648,638, 3,540,396, and 3,780,685. Seadromes of somewhat similar structure are shown in U.S. Pat. Nos. 2,399,656 and 2,399,611. In a preferred tension leg platform shown in U.S. Pat. No. 3,780,685 the buoyant supporting means includes horizontally disposed buoyant members and vertically disposed buoyant members in which the displacement ratio between the horizontal buoyant members and the total displacement of the platform apparatus is between about 0.30 and 0.60 whereby under certain expected wave and weather conditions, vertical force components acting upon the platform apparatus are substantially neutralized and cancelled. A substantially stable tension leg platform apparatus is thereby afforded.